1. Field of the Invention
The present invention relates generally to a method and a composition for applying controlled amount of heat to a base material. More specifically, controlled heating is applied for the purposes of welding, cutting, second material deposition, or surface coating of the base material. An exothermic reaction is used as a heat source which allows for accurate control of the heating temperature and other parameters of the process. Self-propagating high temperature synthesis (SHS) is used as a preferred type of an exothermic reaction.
2. Description of the prior art
Controlled heating of a base material such as metal, plastic or ceramic plate is utilized widely in various industries. Typically, controlled temperature, location, and duration of heating are needed for welding of two or more base materials where these materials may have different physical properties. Cutting or breaking of a base material is another general application for localized heating wherein controlled heat weakens or partially removes some of the base material whereby creating the line of separation. In addition, various coating applications require melting of a secondary coating material over the base material in order to fuse them together. Here again. specific controlled temperatures and other heating parameters are required for successful outcome of these operations.
Exothermic chemical reactions are known to be used for these purposes. There are several important advantages in using such chemical reactions: being well understood, they provide a convenient means for controlling the amount of heat generated during the reaction process. Also, direct application of the components of such reaction to the base material allows for easy control over the dimensions of the heating area. Another advantage is that these reactions are usually self-sustaining and do not require outside energy supply after ignition which makes them convenient for applications in the field.
Components of a typical exothermic reaction are prepared in advance and mixed together typically in a powder form. Once the powder is distributed over the treatment area, it is ignited and the process of burning provides the source of heat needed to achieve the desired function. These processes are well known and widely used. However, handling of the powdery material is difficult because of several reasons. First of all, accurate dispensing of the powder can not be achieved easily. Secondly, even distribution of the powder is also hard to control. Thirdly, holding the powder in place requires the surface of the base material to be perfectly flat and horizontal and no outside disturbance such as air movements should occur prior to igniting and during the burning process. All these factors have made practical use of the powders somewhat difficult and more importantly may lead to irregularities in heating temperature or duration along the treatment area. The need exists therefore for other forms of delivery of the exothermic materials that are easier to use and allow for accurate metering and application of the exothermic reaction components. Following are some known examples of controlled heating using the principles of exothermic reaction.
A method of manufacturing of a composite material according to the U.S. Pat. No. 4,468,272 by Donomoto describes the way to exothermically reduce the amount of metallic oxides in a binder by adding an oxidizing element into the metal matrix. Given examples of metallic oxides are silica, zirconia, chromium oxides and others. Examples of the oxidizing agent include lithium, calcium, magnesium, aluminum, and others. Importantly, exothermic materials are used in a powder form and compressed together in a cast mold prior to igniting. Without the support of a mold, it would be difficult to use these materials with good repeatability and consistency.
Electrically conductive exothermic fine particle powder is described in U.S. Pat. No. 5,378,533 by Ota and consists of fine spherical particles of non-magnetic material such as glass or resin plated with a metal such as Pt, Au, Ag, or Ni. In addition to using this material as a powder, an exothermic coating or paste is described as containing the above mentioned powder and a synthetic resin binder such as polyimide resin, amide resin, silicone resins and the like. Although the paste may be made with uniform consistency which is advantageous for accurate dispensing, the paste application process introduces irregularities which effect the heating parameters.
U.S. Pat. No. 5,549,849 by Namura describes the use of exothermic materials in the form of a viscous fluid such as a paste, varnish, or glue. The fluid according to the invention consists of exothermic powdery materials such as a mixture of nickel and chromium spherical particles together with scales of graphite and particles of organic carbon. This mixture is then added to the resin varnish such as silicone, alkyd, or epoxy varnish to form a gluey viscous fluid. Although application of this fluid was easier achieved by brushing it onto the base material surface as compared with the typically used powder application, it is still difficult to provide accurate and even distribution of the fluid along the treatment area and therefore heating irregularities would still be present.
The use of preformed exothermic materials in casting of molten metals is described in U.S. Pat. Nos. 4,767,800 and 5,180,759 by Neu. Various exothermic compositions are mentioned such as containing aluminum, magnesium, or another readily oxidizing metal mixed with an oxidizing agent such as iron oxide, sodium nitrate and the like and with an organic fluorine containing such compounds as polyvinyl fluoride or other fluorocarbon polymers. A particulate refractory filler is also usually added to the mixture. These compositions are used in a particulate form such as granules or a powder. Shape preforming is also generally mentioned in this patent particularly describing a cylindrical shape and a flat board. In these cases, a binder is added to the mixture such as a phenol-formaldehyde or urea-formaldehyde resin or alternately a gum such as gum arabic, sulphite lye, or colloidal silica. These shapes repeat the shapes of the corresponding molds and do not allow for easy cutting and dispensing of the exothermic composition in general use. There are also no provisions for attaching of the exothermic materials to the base material. The need exists therefore for a preformed universal shape of the exothermic material which will provide for accurate dispensing and placement of this material for a variety of general applications.
One particularly useful type of exothermic reaction is known as a Self-Propagating High-Temperature Synthesis (SHS). In this smokeless burning chemical reaction, very high temperatures may be generated which are useful for joining and welding purposes as well as for cutting and controlled breaking of hard materials such as certain metals and ceramics. According to Messler (Joining advanced materials, by Messler R W Jr., in: Adv Mater Process, February 1995, 147 (2) Photomicrographs p. 47-49), the SHS process holds particular promise for joining ceramics and intermetallics, in either monoliths or reinforced forms, to themselves, to one another, or to metals. An SHS reaction is initiated between reactants to form a compound that generates a significant heat of formation. An example is given where elemental Nickel and elemental Aluminum powders are reacted while sandwiched between Alloy 600 end elements. The bond interface between the reaction product and substrate has high integrity, and the product filler has high density.
SHS process is used extensively in other applications. Synthesis of refractory materials is described in a U.S. Pat. No. 4,459,363 by Holt: refractory metal nitrites are synthesized during self-propagating combustion process utilizing a solid source of nitrogen. For this purpose, a metal azide is employed, preferably NaN.sub.3. The azide is burned with Mg or Ca, and a metal oxide is selected from Groups III-A, IV-A, III-B, IV-B, or a rare earth metal oxide. The mixture of azide, Mg or Ca and metal oxide is heated until ignition temperature at which point an SHS process is initiated which in turn forms the metal nitride while starter materials are depleting in the course of SHS reaction. Similar process is described in U.S. Pat. No. 5,064,808 by Merzhanov to produce oxide superconductors. All these processes use powders as a form of a compound materials. Here again, the use of powders does not offer user friendly handling characteristics and leads to difficulties in accurate dispensing of the reaction components.
Exothermic reactions in general and SHS reactions in particular are used in application of coatings. U.S. Pat. No. 4,363,832 by Odawara describes the use of an aluminum powder mixed with iron oxide pressed against the inside of a pipe due to centrifugal forces originated while the pipe is rotated at high speeds. The powder is then ignited to form a protective ceramic lining inside the pipe. The method of applying the mixture is fairly complex since it calls for high speed uniform rotation of the pipe. A simpler method of mixture application and holding against the surface is needed to provide for easy and uniform deposition of the reaction compounds to the surface of the desired object.
Finally, the use of SHS reaction in applying of wear-resistant coatings is described by one of the inventors of the present invention (Sushchenko SA, Properties of Wear-Resistant Coatings Applied by Exothermic Reactions, International Journal of Self-Propagating High-Temperature Synthesis, Volume 2, Number 3, p. 301-305, 1993). Various SHS reactions are described for application of a protective coating to a substrate by igniting a powdery mixture of SHS reaction components.
Overall, the above mentioned prior art references provide for useful applications of exothermic reactions where a mixture of materials typically in a powder or paste form is applied to the base material and then ignited. Practical difficulties are encountered during the application stage. The need exists therefore for a simple, easy to dispense form of such premixed compounds which will allow for optimal and repeatable heating parameters while reducing the time and efforts needed for such application.